You didn't say where you live and building regulations vary by country, state and county (and province, township, parish, arrondissement, canton . . .).
Do the research and get the precise regulations for radon venting in your area. It's likely that the pipe is required to extend above the roof line. Call your local building department and ask directly if you can't find the information.
If your system is in compliance, you have a choice of leaving it the way it is or hiring someone to extend the pipe above the roofline or doing it yourself if you are able.
If the system is not in compliance, find out if radon remediators must be licensed in your area, and if so, get the name, address and phone number of the regulatory agency for radon remediators.
Now, if the agency supports it, find out if the original installer is licensed now and if he had a valid license when the job was done. Find out the potential penalty for installing radon remediation without a license. Find out the consequences for installing a noncompliant radon remediation system while licensed.
You will then be prepared to address the matter with the installer and deal from a position of knowing which way is up instead of asking a favor of someone who no longer wants to know you.
You don't need to make overt threats, but if you make it known that you know the rules and the consequences, you may get greater cooperation.
